1. Field of the Invention
The present invention relates to an image forming apparatus such as an electronic copying machine in which toner consumed through the forming of an image is supplied automatically.
2. Description of the Related Art
As is known, some electronic copying machines having a developing unit for developing a latent image by use of a developing agent which is a mixture of a magnetic carrier and a nonmagnetic toner comprise an automatic toner supplier which maintains the toner at a constant density in order for consistently high-quality images to be obtained every time the machine is operated.
In such an automatic toner supplier, the permeability of the developing agent in a developing unit is monitored by a toner sensor. When the toner density decreases and the sensor detects that the permeability exceeds a reference value, a control section drives a toner supply motor, to supply additional toner from a toner cartridge to the developing unit. Such an operation continues until the permeability again becomes lower than the reference value level. In the event that the toner density is not restored within a predetermined time period after the toner density has dropped below the reference value level, the control section determines that the supply of toner in the toner cartridge has been used up (toner empty), whereupon it causes a display section to indicate the need for toner replenishment (toner supply) and stops copying from being performed.
The toner sensor detects the toner density from the reactance of the developing agent which varies in accordance with the mixing ratio of the carrier to the toner. The toner sensor has one magnetic member and two magnetic transformers. Each transformer is comprised of a core and multi-turn coils wound around the core. The first transformer (detection side transformer) is located in contact with the developing agent. The magnetic member is located near the second transformer such that it causes the second transformer (reference side transformer) to generate an electromotive force which corresponds to the reference value of the toner density. When the toner density, i.e., the reactance of the developing agnet, changes, the permeability of the core of the first transformer also changes, thus causing a change in the electromotive force induced by the second transformer. The toner sensor is designed to output an analog voltage corresponding to a difference between the electromotive forces of the detection and reference side transformers. With this arrangement, the control section can determine the current toner density with respect to the reference toner density in accordance with the value of analog output voltage from this toner sensor.
When the toner density is controlled by means of such a toner sensor, the analog output voltage from the toner sensor must be initialized to a reference voltage corresponding to the reference toner density of a developing agent. In a conventional automatic toner supplier, this output voltage is adjusted by way of the operator manipulating a volume control knob or the like to alter the position of a magnetic member arranged near the core of a reference side transformer and thereby change the permeability of the core. Not only is this adjustment method cumbersome, but it is also very time-consuming.